1. Field of the Invention
The present invention relates to improvements in an electrode structure in a III group nitride semiconductor device which is represented by a semiconductor laser diode, for example.
2. Description of the Background Art
A GaN type compound semiconductor, which is a III group nitride semiconductor, represented as InxGayAlzN (for x+y+z=1; 0≦x<1; 0<y≦1; 0≦x<1) has a large energy bandgap and high thermal stability, and its bandgap width can be controlled by adjusting the composition. Therefore, the GaN type semiconductor is expected to be a material which is applicable to various semiconductor devices including a light emitting element and a high temperature device. Especially for light emitting diodes (LEDs) using a GaN type material, devices having a luminous intensity on the order of several candelas in a light wavelength range from blue to green have been already developed and put to practical use. Now, the goals of research and development are shifting to obtaining an LED for long wavelength light to provide a full-color LED display, obtaining a laser diode (LD) for practical use employing a GaN type material, and so on.
FIG. 7 schematically shows in section a structure of a p-type electrode conventionally used in a semiconductor device employing a GaN type material. In the p-type electrode, a metal layer 502 of Ni is deposited on a contact layer 501 of p-type GaN and they are annealed at 500° C. in a nitrogen atmosphere for 10 minutes. Thus, an intermediate layer 504 is formed by diffusion reaction between GaN layer 501 and Ni layer 502. On Ni layer 502, a surface electrode layer 503 for wire bonding or device mounting is also stacked. For example, Au is often used as a material for surface electrode layer 503.
In such an electrode structure, intermediate layer 504 has an effect of mitigating a Schottky barrier which is caused when p-type GaN layer 501 and Ni layer 502 are in direct contact with each other.
However, in the p-type electrode on the p-type GaN contact layer according to the conventional art as illustrated in FIG. 7, the ohmic characteristic is unstable and the specific contact resistance value is relatively high and on the order of 10−2 Ωcm2. For example, the specific contact resistance value necessary for a semiconductor laser p-type electrode is about 10−3 Ωcm2 or less. This is difficult to be attained through the conventional art.
The inventor conducted a detailed examination of the p-type electrode structure according to the conventional art. As a result, it was found out that the main component of intermediate layer 504 formed in FIG. 7 is a compound of Ga and Ni (Ga—Ni compound: hereinafter, a compound of elements X and Y is denoted as X-Y compound).
It was also known that the characteristic of intermediate layer 504 is easily influenced by the surface state of GaN layer 501, the degree of progress of interface reaction between GaN layer 501 and Ni layer 502, the annealing temperature and so on, and that obtaining a stable constant electrode characteristic is difficult in the p-type electrode including such intermediate layer 504. It was also found out that especially when intermediate layer 504 is insufficiently formed, the adhesion strength between the p-type electrode and p-type GaN contact layer 501 declines substantially, and the electrode is frequently peeled in providing wire bonding for electrically connecting the semiconductor device to a stem and the like.
It also became clear that a compound of Ni—N is also formed besides the compound of Ga—Ni as the main component inside intermediate layer 504. The source of N for the Ni—N compound is p-type GaN layer 501. In other words, it was also found out that N atoms in GaN layers 501 are absorbed into intermediate layer 504, a portion near the surface of p-type GaN layer 501 is changed to a high resistance layer (or an n-type layer) and, as a result, the p-type electrode structure comes to have higher resistance.